Extra-terrestrial Volcanoes

Earth is not the only place in the solar system with volcanoes!

Click below to learn more!

Mercury

A NASA spacecraft’s first flyby of Mercury has yielded a wealth of information about the inner-most planet, some of which confirms volcanism occurred there, settling a longstanding debate.

MESSENGER images of the Caloris basin, the youngest known impact basin on Mercury, showed smaller craters within the impact basin that had been in-filled with material, “and if you had impact melt [as with the lunar breccia], that wouldn’t happen,” explained Scott Murchie, a co-investigator for the Mercury Dual Imaging System (MDIS), of Johns Hopkins University.

The small craters likely were the result of impacts in the basin long after it was formed and later still, volcanic eruptions spewed lava across the basin, all but erasing the smaller craters. Head says this is “clear evidence that you’re looking at lava flows.”

Venus

Overview

Venus has more volcanoes than any other planet in the solar system. Over 1600 major volcanoes or volcanic features are known (see map), and there are many, many more smaller volcanoes. (No one has yet counted them all, but the total number may be over 100,000 or even over 1,000,000). These volcanoes come in a variety of forms. Most are eitherLarge ShieldsorSmallerShield volcanoes, but there are also manyComplex Features, severalUnusual Constructs, and a fewLarge Flow Features. None is known to be active at present, but our data is very limited. Thus, while most of these volcanoes are probably long dead, a few may still be active.

Background

Venus is like the Earth in many ways. It is nearly the same size and it has a similar bulk composition. Of all the planets, its orbit around the sun is the closest to EarthUs orbit. It has both clouds and a thick atmosphere. Like the Earth, it even has a fairly young surface age (~500 million years). Despite these basic similarities, however, Venus differs greatly from the Earth in detail.

First, since the atmosphere is mostly CO2, Venus has an extreme Greenhouse Effect. In fact, the surface temperature on Venus is about 470!C (about 880!F). Further, the surface air pressure on Venus is about 90 times greater than that at sealevel on Earth. This is roughly equivalent to the WATER pressure on Earth one kilometer beneath the oceanUs surface. These surface conditions have two effects. (1) There is no water on the surface of Venus. Indeed, there is almost no water in the air, either. The clouds are mostly made of sulfuric acid and they are much, much higher than most clouds on the Earth. (2) Due to the high atmospheric pressure, the winds on Venus are also relatively slow. Thus, neither wind nor rain can really affect the surface on Venus. As a result, volcanic features will look freshly formed for a long time.

Second, Venus shows no evidence for plate tectonics. There are no long, linear volcano chains. There are no clear subduction zones. Although rifts are common, none look like the mid-ocean ridges on Earth. Also, continent-like regions are rare, and show none of the jigsaw fits seen on Earth. Thus, where volcanism on Earth mostly marks plate boundaries and plate movements, volcanism on Venus is much more regional and much less organized.

Third, volcanism on Venus shows fewer eruptive styles than on the Earth. Almost all volcanism on Venus seems to involve fluid lava flows. There is no sign of explosive, ash-forming eruptions on Venus, and little evidence for the eruption of sludgy, viscous lavas. This may reflect a combination of several effects. First, due to the high air pressure, venusian lavas need much higher gas contents than Earth lavas to erupt explosively. Second, the main gas driving lava explosions on Earth is water, which is in very short supply on Venus. Lastly, many viscous lavas and explosive eruptions on Earth occur near plate subduction zones. Thus, the lack of subduction zones should also reduce the likelihood of such eruptions on Venus.

Large Shield Volcanoes

Overview

Venus has over 150 large shield volcanoes. These shields are mostly between 100 and 600 km across, with heights between about 0.3 and 5.0 km. The largest shields, however, are over 700 km in diameter and up to 5.5 km in height. For reference, Mauna Loa is ~120 km across at its base, and it has a total height of ~8 km (from the sea floor). Thus, the large Venusian shields are broader, but much flatter then the largest shield volcanoes on Earth. Indeed, the largest shield volcanoes on Venus cover nearly the same area as Olympus Mons, which has a basal diameter of ~800 km. (Note, Olympus Mons is still much bigger than the Venusian shields due to its immense height.)

These large shields all look much like shield volcanoes on Earth. They are mostly covered by long, radial lava flows. They all have very gentle slopes. And most also have some form of central vent or summit caldera. Thus, we think that these shields forme d from basalts, much like the shield volcanoes in Hawaii. The venusian shields, however, show a map pattern which is quite different from that seen on Earth (see reference map). Namely, the shields on Venus are widely scattered, and they show no linear vo lcano chains like those on Earth. This suggests that Venus does not have active plate tectonics, and also that most volcanism on Venus is related to mantle hotspots.

Distribution Map

This map shows the distribution of large shield volcanoes on Venus. Each red triangle marks the site of a shield volcano over 100 km in size. The background shows local surface heights across the planet. Blues denote smooth lowland plains. Greens mark reg ions 1-2 km higher than average for the planet. Reds and yellows show highlands over 3-4 km higher than the rest of the planet.

Very few large volcanoes are in the lowland plains or in the few highland regions. Rather, most of the large shields lie in the green to light yellow regions. Also note how the volcanoes are clustered into left and right sides of the map (dotted lines). T hese regions contain 2 to 4 times more volcanoes (for their size) than the planet as a whole. The reason for this grouping is not fully known, but it may reflect a cluster of closely spaced hotspots in a region of marked crustal failure. This region conta ins a large network of crisscrossing rifts and major fault zones. (Volcano distribution from data of Head et al. (1992); base map shows cycle 1, 2, 3 Magellan topography data.

Sif Mons Volcano

This is a perspective view of the volcano Sif Mons. This volcano is about 2 km high and nearly 300 km across. It lies near the equator and is in a region called Western Eistla Regio. Note: Heights in this image are exaggerated to make the volcano stand ou t, but Sif still shows a very flat profile. This is true for most volcanoes on Venus. Also note the bright lava flows coming to the front of the image. These flows are probably very rough and blocky like aa lavas on Earth. They also are roughly 120 km lon g. To flow so far on such gentle slope suggests that these lavas were also very fluid. (Magellan Press Release Image P-37342.)

Sif and Gula Mons

This is a perspective image of Western Eistla Regio showing two large shield volcanoes. Gula Mons is on the left, and is the largest of these shields. It is ~3.2 km high, and about 400 km across on its biggest side. Sif Monslies in the distance on the right, and is only slightly smaller. Western Eistla Regio itself is broad dome nearly 1.5 km high and over 2000 kmwide. Similar domes are found near hotspots on Earth, and are believed to be lifted by a plume of hot rising mantle rocks. Thus, Sif and Gula seem to be hotspot volcanoes like those in Hawaii.

Where Hawaii is part of a long chain of volcanoes, however, Sif and Gula are far away from each other (over 700 km). They are also far from the nearest other shields. Gula formed at the crossing of two major rift zones, and Sif Mons may also lie on a simi lar rift. Thus, since many other large shield volcanoes also lie near large rift zones, hotspot magmas may need large crustal faults to build big volcanoes on Venus.

The arrow points to a large corona (link to Other Structures) north of Gula Mons. This flat-topped feature is 225 km across and about 600 m high. A few lava flows are seen coming from its nearer edge. (Magellan Press Release Image P-38724, JPL image MGN-7 2.)

Sapas Mons

This colorized image shows the volcano Sapas Mons. Sapas lies near the equator in Atla Regio, and is nearly 180km away from Sif and Gula Mons. Sapas is 1.5 km high and is about 120 km across. It is located on the edge of broad regional dome 1000 km across and up to 3 km high. Unlike many large shields on Venus, Sapas does not lie on a major rift structure, but it does still bury a set of NW-SE trending faults. In the upper right, lava flows also clearly embay two impact craters on the edge of the shield (a rows). Sapas shows many narrow lava flows with a range of brightnesses. Radar is brightest over rough, blocky surfaces; thus, this range of color suggests a change in volcanism over time. Namely, the youngest lavas are fairly bright, but the older lavas seem to be much smoother, especially in the band of dark lavas along the lower right edge of the volcano. (Magellan Press Release Image P-38360, JPL image MGN-51.)

Ushas Mons

Ushas Mons lies in the southern hemisphere on the northern part of Dione Regio. It is on the edge of several lowland plains regions, but it also is on a broad, low rise attributed to a mantle hotspot. Ushas is over 500 km across, and is about 2 km high. L ike Sapas, it shows a number of bright, rough lava flows, but it has smooth dark summit region. It is surrounded by volcanic plains with many small shields (especially in the upper left and lower right). Also note the clear north-south faults running unde r the volcano. These seem to have formed with the volcano, and may contain volcanic dikes. (Magellan Press Release Image P-42386, JPL image MGN-117.)

Theia Mons/Devana Chasma

Theia Mons is the largest volcano on Venus. It is located in Beta Regio, and is shown here in the bottom center. Theia is over 4 km high, and its lava flows cover an area more than 800 km wide. It has an oval central caldera roughly 75 km long and 50 km w ide. The volcano also lies at the junction of 3 major rifts, one of which is seen here running off the top of the image. This rift, Devana Chasma, is over 200 km wide, but it narrows down to ~50 km as it climbs Theia Mons. It is up to 3 km deep on the fla nks of the volcano, and apparently formed throughout the growth of Theia Mons. Also note the crater in the top center (arrow). This crater has been cut by the edge of the rift, and so suggests a fairly young age of faulting.

Small Shields on Venus

Smaller Shields on Venus

Overview

Small shield volcanoes are very common on Venus. In fact, there may be over 100,000 of such shields less than 20 km across. These small shields usually occur in clusters, called shield fields. Over 550 shiled fields have been mapped (see Reference Map), and most of these are between 100 and 200 km across (nearly the same size as the state of Rhode Island). The shield fields are widely scattered on Venus, but they mostly occur in the lowland plains and lower upland regions. Very few are found at higher elevations or in the chaotic tessera.Many have also been partly buried by later lava plains. Thus, the shield fields seem to be fairly old, and they may have formed during the earliest stages of plains volcanism.

At slightly larger sizes, true shield volcanoes are rare. Of the 270 volcanoes between 20 and 100 km in size, only about 70 seem to be real shields. The rest are unusual features that may reflect different lava types. Most of the shields are also fairly small, less than 30-40 km across, and 25 of them show an unusual, petal-like set of bright lava flows (see images). Thus, there seem to be two distinct scales of volcanism on Venus: large and very small. These two scales may reflect differences in the nature or volume of mantle melting beneath the volcanoes.

Reference Map

This map shows the placement of large shield volcanoes, smaller (20-100 km) shields and small shield fields on Venus. It also maps the locations of volcanoes which have "Anemone" or flower-like lava flow patterns. Note the relative number of large shields and of shield fields, and the relative rarity of the smaller shields.

This map also shows two clear patterns of volcanism. (1) Very few of the smaller volcanoes lie at high elevations (light green to yellow). Rather, they mostly lie at lower elevations which are colored blue and dark green. (2) Many of the shield fields also seem to cluster around larger shield volcanoes. This may reflect a concentration of volcanic activity in these regions. However, the lack of shield fields in at least some regions may simply mark their burial by thick lava plains. (Data from Head et al, (1992) J. Geophys. Res. vol. 98, p. 13,153; base map of Magellan Topogaphy from NASA JPL .)

Small Volcano

Single small shield volcano on Venus. This volcano is located in a region of fairly smooth lava plains. Note the clear summit pit/caldera, and the steep-sided cone. Differences in slope angle and the off-centered summit pit are mostly due to foreshortening in the radar data. The bright tail to the right of the shield is probably not volcanic. Rather, this tail marks a rare windstreak left by surface winds blowing around and over the volcanic shield (Magellan Press Release Image P-38810).

Anemone 1

An example of an "Anemone" shield. It lies in southern Atla Regio, and is nearly 40 km across. The name comes from the narrow, radar bright lava flows ringing the volcano. In this example, these flows are buried by a smoother, central unit which contains a central caldera. Outside the volcano, several other bright lava flows are also seen. These flows are part of a broader volcanic complex, but they help to show off and enhance the features of this volcano. (Part of Magellan F-MIDR 10S200, centered on ~9.5S, 201E)

Anemone 2

Shield Field 1

This image shows a typical shield field on Venus. It is about 120 km across, and it contains 51 clear shields of varying heights. There are also ~31 pits that may mark additional vents within the field. Note that the shields show no relation to the older faults in this region, while the pits seem to line up with a large graben (arrows) crossing the field. This suggests that the faults may have partially influenced the type and location of eruptions within the field. For reference, this shield field is located in Ulfrun Regio, about 3000 km east of Sapas Mons. (Part of Magellan C1-MIDR 30N225.)

Shield Field and Flows

This image shows a shield field at the center of a larger set of lava flows. These shields range from ~1 to ~8 km in size, and most occur in a cluster ~80 km across. Some shields occur up to 60 km outside of this cluster, however, while the lava flows extend up to 200 km from the central shield field. Since each small shield is much smaller than any of the lava flows, this shield field must indicate a change in volcanism over time. Most likely, the lava flows formed quickly during an early phase with lots of lava, and the shields then formed later when the magma supply was almost gone. (Part of Magellan C1-MIDR 15N317, centered at 13.5N, 314.5E.)

Shield Field and Corona

This image shows a shield field inside a corona structure. This field is roughly 125 km across, and it is surrounded by a set of younger lava plains. Note the dark unit which buries both fractures and shields in the lower left. In this case, the shield field may have formed as the first magmas slowly came up inside the corona, while the plains would have formed later during a more vigorous phase of volcanism. (Part of Magellan C1-MIDR 30N135, centered at 27.5 N, 136.5E.)

Large Lava Flows

REFERENCE MAP (Channels, Flows, Calderas)

Large liquid lava flows are thought to form three types of feature on Venus. The first are large, flood-like lava flow fields or fluctus. The second are long lava channels. And the third are large calderas that have no clear shields or cones. None of these features are common on Venus, but they are still plentiful and several large examples of each type are known.

Large Flows

The map above shows the locations of some 53 lava flood fields. These flow fields are mostly 100 to 700 km long, but the longest are over 1000 km in length. Their widths are variable, but can reach over 300 km. In size, they are thus much like lava flood basalts on the Earth, and like the youngest mare flows on the Moon. They mostly lie near the edges of the lowland plains, and flow down into these plains. A few are located near younger large shields, but many lie at some distance from the largest shield volcanoes. It is thought that these flows may have formed near the end of plains formation.

Lava Channels

In all nearly 200 channels have been found on Venus. Many of these channels lie in the lava flood fields or on other volcanoes. Still, nearly 50 lava channels have been found which do not lie on a larger lava flow. These channels are usually 0.5 to 1.5 km wide, but they range widely in length. Most are less than 400 km long, but some are over 700 km long. One is longer than every other known lava channel in the solar system. With a length of about 6800 km (~4200 miles), this one channel is even longer than the Nile River on Earth.

Like the Lava Flood Fields, most lava channels start near the edges of the lowland plains. Many are also found near coronas or in large groups. In general, the lava channels on Venus look much like sinuous rilles on the Moon. The longest channels, however, are not quite the same. First, they tend to be straighter and wider than the lunar rilles. Second, some show older branches and cut-off loops like those in large rivers on Earth. This suggests that some of these channels may have formed over a long time from one or more eruptions.

Calderas

The last features mapped here mark sites from which large volumes of lava are believed to have erupted. These calderas are large, round to oval pits that have steep walls but very low rims. They are often surrounded by a ring of faults or graben. Calderas are thought to form when the ground falls in over an emptied, shallow magma chamber. Although part of many shield volcanoes, 86 calderas have also been found on Venus which are not. Most of these calderas are between 40 and 80 km in size, but the largest are nearly 200 km across. Most are near the shields of Atla and Beta Regio, but a few also lie in the high plains of Lakshmi Planum. Almost no calderas are found in the lowland plains. This lack suggests that shallow magma chambers did not form in the lowlands. If this is true, then lavas must have risen straight to the surface there. Thus, the lavas would have little chance to pool together or to build up in a single area over time. This could explain both plains formation and the small sizes of the lowland shields. In the highlands, however, we find both calderas and large shields. Thus, shallow magma chambers could form higher up.

This change in volcanism with height is thought to be an effect of air pressure. In the lowlands, air pressures are very high, nearly 100 times those on Earth. Thus, lowland lavas are much like seafloor lavas on Earth. They are very dense and contain almost no gas bubbles. In the highlands, air pressures are lower, and this lets some gas bubbles form on eruption. This makes highland rocks less dense and can allow lavas to pool below the surface. If the lavas pool in one place over many eruptions, then a large shield can be built up over time. However, no such build up can occur if the pool does not fill up again after the eruptions. Instead, a caldera should form like those mapped here.

Complex Features

Tectono-Volcanic Structures

REFERENCE MAP (Tectonic/Volcanic Features)

Volcanism is also part of some very complex features on Venus. These features mix both lavas and faulting; thus, they are called TECTONO-VOLCANIC structures. They differ from volcanoes in two ways. First, volcanoes often form on older rifts or faults, but they do not cause this faulting. Second, most volcanoes are just large piles of lava. In contrast, the tectono-volcanic structures are thought to form by faulting over rising magmas. Also, lavas make up only a small part of these structures. Due to differences in faulting, three types are found on Venus.

Coronas

Some 200 to 300 coronas are known, of which 175 are mapped above. These are large, round to oval shaped features with a distinct ring of faults or ridges. They often have a flat, raised or down-dropped center and an outer moat-like trough. Lava plains and small shields are found in both the centers and the moats, and pancake domes are very common as well. Coronas range in size from about 100 km to nearly 1000 km, but most are 200 to 250 km across.

Coronas are thought to form over small mantle plumes. First, rising magmas and heat lift the surface. These plumes also feed local eruptions, but they are too small for a long string of eruptions. Thus, the uplifted surface is not fully buried, and a complex mix of faults and lavas is formed. With later cooling, the uplift then sinks to yield the down-dropped centers seen in the oldest coronas.

Arachnoids

Arachnoids are smaller cousins of the coronas. Like coronas, they have a round ring of faults or ridges, but these rings lie inside a set of radial ridges. The rings range from about 50 km to 200 km in size, with the outer ridges running out another 200 to 400 km. Over 250 arachnoids have been mapped, and they tend to cluster near both coronas and other arachnoids. Also, like the coronas, arachnoids are rarely found in the lowest plains. Instead, most lie just above the lowland plains (i.e., the green map areas).

Arachnoids look like coronas and form near coronas. Thus, they are thought to form in much the same way as coronas. They are smaller than most coronas, however, and they tend to show fewer lavas. Thus, they probably formed over smaller plumes. Since smaller plumes should have less magma and should cause less uplift, this model seems to fit the facts. However, the lack of lava flows also suggests that there are more intrusions in arachnoids than in coronas. Indeed, it has been suggested that the radial ridges may be large dikes. In this case, these dikes could drain magmas away from the plume and limit the eruption of lavas at the surface.

Novas

Novas show fewer signs of real volcanism than the coronas or the arachnoids. Instead, they show a starburst-like pattern of faults and a broad, dome- like uplift. Some of these faults seem to feed lava flows, but such flows are not common. About 50 Novas have been mapped, with sizes ranging from about 50 km to 300 km. Most are between 150 and 200 km across, and thus are the same size as many of the arachnoids. Although rare, novas tend to occur near large volcanoes or near groups of coronas and arachnoids. They are seldom found alone or in the lowland plains. Since the higher plains on Venus are thought to lie over mantle plumes, this suggests that novas are linked to mantle melting in some way. Given their size and shape, they may mark an early stage of uplift over small mantle plumes. If this is true, then these novas may turn into arachnoids or coronas in a few million years.

Mylitta Fluctus

Mylitta Fluctus is one of the largest lava flow fields on Venus. It is about 1000 km long (600 miles) by 460 km wide. Thus, it covers an area slightly larger than the state of Arizona (300,000 square km). It lies on the southern edge of Lavinia Planitia, and drops some 2000 meters from south to north. Note the large crater which is partly flooded in the southeast (arrows).

This flow field contains many lava flows. These vary in length from 400 to 1000 km, and form ~30 km to 100 km in width. Many of these flows contain central lava channels like those seen on Hawaii. The flows seem to have formed in 6 separate eruptions, and most come from a single center in the southeast (marked source). This source is a large shield volcano that was formed by the first eruption event. The later eruptions then produced the longer flows of the main flow field. On the basis of Earth lavas, it is thought that the shield formed in about 10 to 70 years. Each of the later lava flow sets could have formed in less than 2 to 80 days.

Note: While Flood Basalts on the Earth are as large as Mylitta Fluctus, most Flood Basalts do not come from a single source. Rather, they come out of long fissures which are buried by the erupted lavas. Thus, the flow of lavas away from the shield here suggests that it may be harder for lavas to reach the surface on Venus. Since the shield lies on a major rift zone, faulting may have helped these lavas reach the surface.(Image from Magellan C2 MIDR 60S333;1, with parts from C2 60S333;202.)

Atla Flow Field

This image shows another lava flood field. This one lies on the edge of Atla Regio, and is also about 1000 km long. At its widest, it is nearly 300 km across, but it also narrows down less than 50 km in some places. In area, it is roughly the same size as state of Oklahoma (~180,000 square km).

Like Mylitta Fluctus, this flow field formed in several stages. Here, however, there is no shield volcano at the source. Rather, the lavas erupted four times from a small group of faults and graben. Like Mylitta, these faults are part of a larger rift system. After eruption, the lavas then flowed west along the edge of Atla Regio. Note how they arc around to follow the lowest ground, and then flow into a smaller rift.(Image from Magellan C1 MIDRs 00N197 and 00N215.)

Meandering Channel

This is one part of a long lava channel in Helen Planitia. In all, the channel has a total length of almost 1200 km, but the segment shown here is only 200 km long. It is also about 2 km wide. Note how the channel snakes along in a band that is slightly brighter than the surrounding plains. This band probably formed from thin lavas that flowed over the channel's banks. In addition, a much older lava channel can also be seen. The marked pair of bright lines (arrows) seems to mark a channel that has almost totally faded away into the nearby plains.

Although well preserved, the main channel also seems to be old. First, its ends fade away into the plains lavas. This suggests that the channel has been buried in places by younger lavas. Second, the channel is cut by a swarm of ridges and faults (see upper center). Probably, the channel formed soon after the local plains, and both then saw nearly 300 million years of slow deformation.(Press Release Image P39226, MGN-82, centered near 49S, 273E.)

Relict Channels

This is part of another lava channel. It lies just south of Ishtar Terra, and also is about 2 km wide. It clearly shows a set of cut-off channels and islands that look much like those seen on some Earth rivers. Thus, it seems that the lavas changed their path over time much like EarthUs rivers. Clear signs of erosion are also seen inside the channel in the upper right. Thus, it looks as if the lavas cut down into older flows. These changes in the flow path are likely the result of later lava flows using an older channel. Still, they might also have formed during one very long eruption.(Image is part of Magellan F MIDR 45N019.)

Braided Channel Segment

Venusian channels also form more complex systems. Here, we see one part of such a system. In the upper left are several channels that formed when lavas spilled out of a fault-bounded trough. These channels merge in the center of the image, and then run into a ridge of highlands. The lavas pooled behind this ridge, before spilling over it too to flow further east. In the process, they carved deep outlets through the highlands (see arrows) and left a number of stream-lined islands.

These islands, at both the left and right, look much like features carved by large floods on the Earth and on Mars. Thus, it is thought that the lavas in this channel behaved much like flood waters on the Earth. Given the slow speed of most Earth lavas, this in turn suggests that the lavas were not basalts. Rather, they may have been very hot mantle melts (komatiites), or possibly liquid sulfur. For reference, this image is about 250 km wide. It shows part of a 1200 km long channel which flows around the Ammavaru volcanic complex. It lies in the south near Lada Terra.(Image is part of Magellan F MIDR 50S021, and is centered near 51S, 22E.)

Sinuous Rilles

This image shows shorter sinuous rilles more like those seen on the Moon. Here, the smallest rilles begin at small or middle-sized round pits. The larger rilles begin at bigger, more complex collapse zones. Note how the rilles narrow away from these sources. This suggests that the lavas slowed and cooled as they moved away from the source vent. Thus, these eruptions were likely smaller and more short lived than the eruptions that formed longer lava channels. Also note how the source pits line up with the older faults in this image. Once again, these faults probably helped control where the lavas could reach the surface.(Image part of Magellan C1 MIDR 15S095, centered near 11S, 89.5E.)

This caldera shows what most calderas on Venus look like. The central hole is about 36 km across, and it is surrounded by a large set of arcuate faults. These form a bulls-eye pattern over 100 km across. Note that these faults also cut a ring of lava flows that formed before the caldera fell in. Inside the caldera, the floor is made of dark, smooth lavas that erupted after collapse. Even after these lavas, however, the floor still rose and fell a few more times. This is shown by the ring of faults inside the floor and by the faults at the caldera's center.

Impact craters also are round holes, but most craters and calderas do not look alike. First, impact craters almost never have a bulls-eye of faults outside their rims. Second, where most impact craters have sharp, raised rims, most calderas have low, rounded rims like that seen here. Third, most impact craters of this size have a clear peak rising out of the crater floor. Calderas mostly have smooth floors. Lastly, calderas often have clear lava flows on either their rims or floors. Impact craters rarely show such signs of volcanism.(Image part of Magellan F-MIDR 05N228.)

Sacajawea Caldera

Sacajawea is one of the largest calderas on Venus. It is roughly 150 km long by 100 km wide, and it seems to be over 1000 m deep. It lies on the high plains of Lakshmi Planum. Again, we see a large ring of fractures outside the caldera, and a smooth lava floor inside the caldera. Some of the floor lavas even bury the ring fractures in the left center. Also note how the fractures are blurred north of the caldera. This may be a region where thin lava flows have buried the older ring faults as well. A number of small domes and shields are also found in this area, and on the southeast rim. All these features suggest that Sacajawea formed from many eruption events. Indeed, Sacajawea sits on a broad (~600 km wide), low rise that may be a very flat shield-like volcano.(Image part of Magellan C1 MIDR 60N319, centered at ~66N, 336E.)

Fotla Corona

This image shows a small, fairly simple corona. This corona has two parts. There is a central plains region roughly 150 km across, and an outer fracture ring about 45 km wide. The central plains stand slightly higher than the outside plains, while the fracture ring seems to lie in a slight trough. There are also many signs of volcanism in the area. For instance, three pancake domes lie close to the fault ring (1,2,3). There is also a string of small shields along the bottom of the image (5). And the fault ring itself is buried by plains lavas in several places (arrows). All the pancake domes are cut by some ring faults, as are the younger plains. Thus, the corona was still forming when these lavas were erupted. This mix of major faulting and volcanism is why coronas are called tectono-volcanic features.

Note: there is a fourth round feature in this corona that is not a pancake dome. Rather, its edge is a set of concentric faults that seem to have formed around a central uplift. Possibly, this marks deformation over a magma intrusion. Or maybe it is somehow related to the faulting in the outer ring.

Idem-Kuva Corona

Idem-Kuva is about 230 km across, and is a more complex corona. It lies just north ofGula Mons, and it shows a multi-ringed plan. At the center is a large plateau, about 100 km wide and 600 m high. This looks somewhat like a pancake dome, but it is much bigger. It probably formed by heating and uplift rather than by eruption. On the edge of the central high are two arcuate troughs, and these are bounded in turn by a partial ring of faults. These faults may have once ringed the entire feature, but they seem to be buried on the north by younger plains lavas. Two long, bright lava flows also run north out of the troughs and over the fault ring. These flows are about 150 and 350 km long, and they mark the clearest signs of volcanism in Idem-Kuva.

This image also shows two other features. First, there is a major line of faults running northwards into Idem-Kuva (arrows). These faults are part of a rift zone on Gula Mons, and they may have fed the two lava flows seen in Idem-Kuva. Second, the pale circle in the upper left marks an older volcanic center (Nissaba). This volcano is fairly large but very flat. It is 300 km long by 200 km wide, but it is only about 400 m high. Note how the leftmost flow out of Idem-Kuva curves around its base. This suggests that Nissaba formed before the flow. It may even have formed before Idem-Kuva.(Image is part of Magellan C1 MIDR 30N351.)

Corona Chain

Many coronas lie in groups or chains. Here, we see one such grouping of 4 or 5 coronas about 200 to 250 km across. It is part of a much longer chain over 4000 km long. The coronas overlap in places, but they are also linked by a number of bright faultlines. These faults are part of a major rift zone, Parga Chasma, that seems to have helped the coronas form. Indeed, the Parga rift has the highest density of coronas to be found on Venus. It also runs into Atla Regio and may have helped to feed the large shields found in that region. Thus, while rifts can bring lavas to the surface, this rift may also lie over a line of deeper mantle melting and uplift.(Image is part of Magellan C2 MIDR 30S284, centered near 34S, 275E.)

A Simple Arachnoid

This arachnoid lies in the plains northwest of Atla Regio. Like a Corona, it has a clear fault ring and a central (mostly) smooth region. Unlike a Corona, it also has a set of radial ridges which arc away from the fault ring. It is also much smaller than most coronas. In this case, the fault ring is about 95 km across, and most arachnoids are about the same size. In contrast, most coronas are over 200 km across. Still, outside the fault ring, the ridges seem to run 100 to 200 km before merging with other ridge systems.

Most arachnoids show few signs of volcanism. Here, there are a few small shields in the center, but the plains lavas show the clearest signs of volcanism. Note the bright lava flows on the northwest and southeast that seem to come out of the fault ring. Also note how the dark plains arc above the arachnoid, and how they ring the crater in the bottom center (arrows). These all suggest that many large, thin lava flows were erupted as the arachnoid was forming.(Image is part of Magellan C1 MIDR 45N223, centered near 41N, 214E.)

Complex Arachnoids

Arachnoids often lie in faulted plains, and their outer ridges tend to turn into regional patterns. Thus, they often look like spiders on a bright web. Indeed, the name Arachnoid means Rspider-likeS. Here, we see two arachnoids in a faulted plains region (A and B). These features are each about 120 km across, with an outer ring of ridges maybe 150 km wide. Again, there is little sign of volcanism. Only a few bright lava flows are seen in the upper left. A third feature (C) may also be an arachnoid. However, it has only a few radial ridges, and it also shows a caldera-like pattern of nested cliffs. Thus, it is harder to classify. The edge of a larger corona (arrows) also can be seen at the top of the image.(Image part of Magellan C1 MIDR 45N011, centered near 40N, 19E.)

Simple Nova

Unlike Coronas and Arachnoids, Novas do not have major ring features. Rather, they show a striking starburst of faults and graben. Also, there is often a clear rise at the center of these faults. This example is some 250 km across, and it lies in Themis Regio. There is no clear sign of volcanism, but a few small lava flows may run out from faults in the upper left and upper right. Still, there is a lot of volcanism nearby. Namely, this nova lies within a very long chain of coronas. Indeed, it lies just above the corona chain image in Parga Chasma. Novas mostly occur next to large shields or major corona chains. Thus, they seem to be volcanic. They probably mark an early stage of uplift and faulting that could later become a corona or a large shield volcano.(Image is part of Magellan C1 MIDR 30S279, centered near 27S, 272E.)

Transitional Form

This image may show the future of other novas on Venus. It shows a complex volcano with several clear parts. Its raised center has a clear starburst of nova-like faults. Around this nova is a partial ring of corona-like faults (arrows). Then, outside the corona, is a broad apron of thin bright lava flows. These flows lie on darker lavas which, in turn, clearly bury a much older area of heavily faulted lowlands. In all, the outer lava flows mark the start of a shield volcano some 450 km long by 350 km wide. The central nova itself is about 150 km across. Maybe, in 10 or 50 million years, more flows will finally bury the central nova to make a shield looking like Ushas Mons.(Image part of Magellan C1 MIDR 15S215, centered at 15S, 215E.)

Unusual Volcanoes

Unusual Volcanoes on Venus

Overview

Most volcanoes on Venus are shields, but a few are not. These volcanoes all formed from very thick, viscous lavas, and they fall into three types. First are the so-called "pancake" domes. Second are a related class of "tick-like" structures. And third are a few volcanoes with thick, fan-shaped or banded flows. Since most basalt lavas are very fluid and fairly thin, these volcanoes do not seem to be made from basalt. Rather, they may mark quartz-rich or granitic lavas. Still, there are theories that gas-rich, "frothy" basalts could produce such viscous lavas. Also, few of these features lie in or near the Venusian highlands. Thus, these three volcano types may differ greatly from the volcanoes seen on Earth's continents.

Distribution Map

Most of the non-shield volcanoes on Venus are pancake domes, shown here in yellow. Like the shield volcanoes, these domes are widely scattered. They also often form small groups or clusters. Where most venusian shield fields have well over 20 small shields, however, few dome clusters have more than 5 or 6 domes. Further, the pancake domes are rarely found near shield volcanoes on Venus. Instead, many domes lie near corona structures in the lowland plains.

The ticks, shown here in red, show a pattern similar to the pancake domes. Like the pancake domes, most ticks occur in the lowland plains far from the largest shields on Venus. Many are found near the lowland coronas, and some are quite close to one or more pancake domes. The ticks also look something like the pancake domes. Thus, it is thought that many ticks are modified dome features.

Finally, fan-shaped and banded flows are quite rare. They are shown on this map in purple, and most are located in one part of the southern lowland plains. Neither their setting nor their distribution shows any clear trends, so little is known about these flows. Still, they look a lot like the most viscous lava flows on Earth. Thus, they may not have formed from basalt lavas. Instead, they may mark a rare set of granitic or rhyolitic lavas on Venus.

Domes in Eistla

This image shows three of the venusian pancake domes. They lie near the equator on the edge of Eistla Regio. The two larger domes are roughly 65 km (40 miles) wide, and the smaller dome is about 22 km (13 miles) wide. All three are very low, rising less than 1 km above the surrounding plains. The tops are fairly flat to slightly bowl-shaped, and they show clear patterns of small cracks and faults. Most of these cracks probably formed with the dome, but some may have formed later with the faults in the surrounding plains. Note how some features extend from plains onto the edges of the central and right hand domes (arrows).

Each dome also shows a small central pit. These pits look like the central vents found on shield volcanoes, but they seem to have formed after their respective domes. Thus, there is no sign that the central pits fed lava flows to the dome. Rather, the domes seem to have formed from one large, slow eruption of viscous lava. After the eruption, the central pits then formed on the dome as gasses escaped from the magma or as the dome cooled and shrank. (Magellan Press Release Image P-38388, JPL image MGN 53.)

Domes near Alpha

This image shows a chain of pancake domes east of Alpha Regio. Each dome is roughly 25 km (15 miles) wide, and the highest are about 750 m tall. As in Eistla Regio, most of these domes show a (very) small central pit. Here, however, we see a variety of crack patterns on the domes. These probably mark small changes in the speed or conditions of eruption. Many of the domes here also overlap. Such overlaps are quite common, and most seem to mark a series of dome eruptions. In some cases, however, they may mark two domes that formed at the same time. (Magellan Press Release Image P-37125).

The So-Called Tick

This feature is called the "Tick" for obvious reasons. It lies near the edge of Eistla Regio and is roughly 66 km (41 miles) across. The central round "body," however, is only about 35 km (22 miles) wide. Like the pancake domes, the ticks are broad, mostly flat features, and they often have a central pit or vent structure. The difference is that the ticks are surrounded by an array of short, radial ridges. In this case, the "head" of the tick is defined by a set of small collapse pits.

The origin of the "leg" ridges is unknown, but two options have been suggested. First, the ridges may outline avalanche scars. In this case, the tick is simply an old pancake dome with a heavily eroded rim. The second option is that the ridges mark dikes running out from the central "body." Still, it is not clear why these dikes formed ridges and not lava flows. Thus, some rim erosion may also be needed to expose the dikes in this theory. (Part of Magellan FMIDR 20S003, centered near 18.5S, 6E.)

Another Tick

This image shows another "tick" south of Aphrodite Terra. It is about 12 km wide, and it lies in a region of deformed plains lavas. Note the fractures which arc around this tick to form an eyelike feature. Since this feature is about 44 km long by 33 km wide, it suggests that tick formation may affect a fairly broad region. Also note the crinkled region above the eye in this image. This seems to be a lava flow that came out of the tick before the latest stage of local faulting. (Part of Magellan C1MIDR 30S189, centered near 29.5S, 184E.)

An Unusual Volcano

This strange volcano lies in Aino Planitia. Its central dome is roughly 100 km across, but it is only about 1 km high. Around the dome are several thick, fan-shaped lava flows and a much larger flow with a banded surface. These flows strongly suggest the eruption of very viscous lavas. First, the fan-shaped flow lobes are very thick. The measured flow heights vary between 120 and 540 m (~400 to 1700 feet). Second, the shape of these flows suggests that the lavas had a hard time flowing away from the volcano. Third, while the larger flow is much thinner, it looks a lot like the banded flows produced by viscous lavas on Earth.

Viscous lavas on Earth are rarely basalts. Hence, this volcano may mark a rare case of non-basaltic lavas on Venus. Such lavas need a lot of water to form, however, while water is scarce on Venus. Further, basalts can also form viscous lava flows in some cases. Thus, it is not yet clear what the nature of these lavas was. (Magellan Press Release Image P-39916, JPL image MGN-93).

Earth's Moon

Volcanism on the Moon

By Robert Wickman

Overview:

The Earth's Moon has no large volcanoes like Hawaii or Mount St. Helens. However, vast plains of basaltic lavas cover much of the lunar surface. The earliest astronomers thought, wrongly, that these plains were seas of lunar water. Thus, they were called " mare " (pronounced "mahr-ay"). Mare means "sea" in Latin. In addition, other volcanic features also occur within the lunar mare. The most important are sinuous rilles , dark mantling deposits,and small volcanic domes and cones . Most of these features are fairly small, however. They form only a tiny fraction of the lunar volcanic record.

1. Oceanus Procellarum

2. Mare Imbrium

3. Mare Cognitum

4. Mare Humorum

5. Mare Nubium

6. Mare Frigoris

7. Mare Serenitatis

8. Mare Vaporum

9. Mare Tranquillitatis

10. Mare Nectaris

11. Mare Humboldtianum

12. Mare Crisium

13. Mare Fecunditatis

14. Mare Marginis

15. Mare Smythii

16. Mare Australe

17. Mare Moscoviense

18. Mare Ingenii

19. Mare Orientale

Differences from Earth:

Volcanism on the Moon differs in several ways from volcanism on the Earth. First, there is the matter of age. Volcanism on the Earth is an ongoing process. Many of Earth's volcanoes are quite young in geologic terms, often less than a few 100,000 years old. In contrast, most volcanism on the Moon appears to have occurred between 3 and 4 billion years ago. Typical mare samples are ~3,500,000,000 years old. Even the youngest mare flows have estimated ages of nearly 1 billion years. These "young" rocks have not been sampled or directly dated, however, so this age is very poorly known. For comparison, the oldest dated rock on the Earth is ~3.9 billion years old. The oldest sea floor basalts on Earth are only about 200 million (0.2 billion) years old. Because the Moon does not show any evidence for recent volcanic or geologic activity, it is sometimes called a "dead" planet.

The settings of mare volcanism reveal another major difference from volcanism on the Earth. Specifically, Earth's volcanoes mostly occur within long linear mountain chains. Mountain chains like the Andes mark the edge of a lithospheric plate. Mountain chains like the Hawaiian Islands mark past plate movements over a mantle hotspot. In contrast, the mare typically occur in the bottoms of very large, very old impact craters. Thus, most of the mare are nearly circular in shape. Further, lunar mountain chains form the edges of these impact basins and tend to surround the lunar mare. There is no evidence that any system of plate tectonics ever developed on the Moon. Finally, the lunar mare are primarily found on one side of the Moon. They cover nearly one third of the lunar nearside (see figure), but less than 2% of the lunar farside. The surface is much higher on the farside, however, and the crust is typically much thicker there as well. Thus, the primary factors controlling volcanism on the Moon appear to be surface elevation and crustal thickness.

Finally, there are some major physical differences between volcanism on the Earth and on the Moon. First, lunar gravity is only one sixth that of the Earth's. This means that the forces driving lava flow are weaker on the Moon. Thus, the very flat and smooth mare surfaces imply that mare lavas were very fluid. They could both flow very easily and spread out over large areas. Also, the low gravity means that explosive eruptions can throw debris further on the Moon than on the Earth. Indeed, such eruptions on the Moon should spread lavas out into a broad flat layer and not into the cone-shaped features seen on the Earth. This gives one reason for why large volcanoes are not seen on the Moon. Second, the Moon has essentially no dissolved water. The lunar mare are all bone dry. In contrast, water is one of the most common gases in Earth lavas. Water also plays a major role in driving violent eruptions on the Earth. Thus, the lack of lunar water should strongly affect lunar volcanism. In particular, without water, violent explosive eruptions are much less likely on the Moon. Instead, lavas should just flow smoothly and quietly out onto the surface.

Mare

Due to their size, the lunar maria are the most obvious volcanic features on the Moon. These vast basalt plains cover over 15% of the lunar surface, mostly on the Moon's nearside. They are typically circular in outline because they tend to fill the bottoms of very large, very old impact basins. Smaller mare patches also occur in the floors of some impact craters. They also are very old, and have been battered by the impacts of many small meteorites for over 3 billion years.

Major Lunar Maria

1. Oceanus Procellarum

2. Mare Imbrium

3. Mare Cognitum

4. Mare Humorum

5. Mare Nubium

6. Mare Frigoris

7. Mare Serenitatis

8. Mare Vaporum

9. Mare Tranquillitatis

10. Mare Nectaris

11. Mare Humboldtianum

12. Mare Crisium

13. Mare Fecunditatis

14. Mare Marginis

15. Mare Smythii

16. Mare Australe

17. Mare Moscoviense

18. Mare Ingenii

19. Mare Orientale

Shown here is a map of the major lunar maria. These maria range from over 200 km to about 1200 km in size. They are typically about 500 m to 1500 m thick. However, each mare appears to contain many thinner basalt flows. Typical flow thicknesses appear to be 10-20 m. Thus, each mare records hundreds of overlapping eruption events. The map also shows a clear lack of major maria on the lunar farside. This probably reflects two changes in the lunar crust. First, the lunar surface is higher on farside than on the nearside. Second, the crust seems to be thicker on the lunar farside than on the nearside. These differences should make it harder for mare magmas to reach the surface on the lunar farside. They also explain why small mare patches are grouped together on the farside. The mare patches represent lava-filled craters. Most such craters lie in the bottoms of much larger and much older basins. On the nearside, such basins contain circular mare. On the farside, such basin filling volcanism is rare. Still, these basins contain both the lowest surfaces and the thinnest crust. Thus, mare volcanism is most likely inside these basins, especially where younger craters have dug into the basin floor. (Map prepared by G.W. Colton; published in NASA SP-362 (1978) and NASA SP-469 (1984).)

Mare Surface

This is an Apollo photo of the surface in southern Mare Imbrium. It shows some young, fairly pristine mare lava flows. These lavas are probably 1 to 2 billion years old. Still, individual flow lobes can be clearly seen at the top of the image. Similarly, the gully-like features in the lower left do not mark any kind of erosion. Rather, they mark shallow lava channels (sinuous rilles) which formed at the lava flow surfaces. The image also shows many small circular impact craters. While meteorite impacts on the Earth and Moon are rare, such craters are quite common within the lunar mare. The mare are so old that a large number of meteorite impacts have occurred. Indeed, the number of impact craters within a mare provides a method for guessing its age. Because older surfaces are more likely to have been hit by meteorites, older mare should contain both more craters and larger craters than younger mare. Note -- Younger mare lavas can bury craters formed on older lavas. This image shows one such example near the crater in the bottom center. The rough ejecta unit surrounding this crater is cut and partially buried by younger lava flows. (Mosaic of Apollo photographs A17 M-2295 and A15 M-1701)

Mare Humorum

This image shows Mare Humorum and the western edge of Mare Nubium. Mare Humorum is a small circular mare on the lunar nearside. It is about 275 miles (~440 km) across. The mountains surrounding Mare Humorum mark the edge of an old impact basin. This basin has been flooded and filled by mare lavas. These lavas also extend past the basin rim in several places. In the upper right are several such flows which extend northwest into southern Oceanus Procellarum. Note the large fractures arcing around Mare Humorum on the right. These fractures are believed to mark a bending of the lunar surface due to the weight of Mare Humorum. Such a sinking of the mare may also explain the two large, partly flooded craters that seem to slope into Mare Humorum. (Earth-based telescopic photo from the Consolidated Lunar Atlas)

Mare Moscoviense

This is an oblique image of the lunar farside. It shows the impact basin that holds Mare Moscoviense. Like Mare Marginis, this mare appears to be fairly thin. However, it is clearly centered within a large impact basin. It is also much lower than either the outer basin floor or the farside highlands. The great depth of this mare beneath the nearby highlands probably explains why mare units are so rare on the lunar farside. Very few basins on the farside were deep enough to allow mare volcanism. Such a contrast in mare and highland elevations also exists on the nearside. Still, it is much smaller than that found on the farside. This may be because the Moon's crust is much thinner on the nearside. Thus, while large impact basins are found on both the nearside and farside, large maria are mostly found on the nearside. Mare lavas apparently could reach the surface more often and more easily there. (Lunar Orbiter image IV-103-M)

Mare Marginis

Mare Marginis lies on the very edge of the lunar nearside. Thus, it lies halfway between the lunar nearside and farside. It also differs from most of the nearside maria. It has an irregular outline, and it appears to be fairly thin. Note the small circular and elongated features in the mare plains. These probably mark impact craters buried by less than 1000 - 1700 feet (300-500 meters) of lava. Further, Mare Marginis is not centered on any clear, large impact basin. Thus, Mare Marginis seems to mark a low-lying region of the highlands where mare lavas were just able to reach the surface. Several large mare-floored craters also occur nearby. In these craters, the crater floors lie below the surrounding highland surface. Thus, they mark sites around Mare Marginis where lavas were close to the lunar surface. (Lunar Orbiter image IV-165-H3)

Imbrium Flow Map

Lava flows within the lunar mare are quite large. Shown here is a map of 3 "young" lava flows in Mare Imbrium. These flows apparently record three separate eruptions within a period of ~500 million years over 2.5 billion years ago. The oldest group is the largest. Its furthest point lies about 750 miles (~1200 km) from the inferred vent in the lower left corner. The second group then buried parts of the first group. It extends for a distance of about 375 mile (~600 km). Finally, the youngest group is also the smallest. It is ONLY some 250 miles (400 km) in length. (NOTE: some areas contain a mixture of flows from the first and second flow groups and mapped here as "mixed.") No active Earth volcanoes have lava flows anywhere near this length. Still, a few older eruptions are of similar size. Due to their size, these features are called Flood Basalts. One example is the Columbia River Flood Basalts in the northwestern U.S. They extend from Idaho into the Pacific Ocean. Most of these flows formed about 16 million years ago, but some erupted as recently as 6 million years ago. The biggest flows are over 188 miles (300 km) long, and they collectively cover over 102,500 suqre miles (164,000 square kilometers). Thus, the Columbia River Basalts are nearly the same size as the youngest and smallest of the basalt flows in Mare Imbrium. (Map after figure 4.26 in the Lunar Sourcebook; based on Schaber, 1973)

Types of Mare Basalt

The lunar mare are very dark when seen with the naked eye. They are not all of the same color, however. Small differences are present in the amounts of ultraviolet, visible and infrared light reflected from the mare. Such color differences define 13 mare basalt types (shown here). These basalt types should mark changes in the minerals and chemistry of the mare basalts. However, the exact nature of over half of these mare units is poorly known. Most are located far from the Apollo landing sites. We have samples for only the 4 basalt types labeled Apollo 11, Apollo 12, Apollo 15, and Luna 20. Note -- The mare reflect only a small fraction (~7-10%) of visible light. Thus, most of the color differences in this map are invisible to the human eye. (Figure from Pieters (1978) Proceedings of 9th Lunar & Planetary Science Conf., vol. 3, p. 2826.)

Sinuous Rilles

Sinuous rilles are probably the most recognizable of small volcanic features on the Moon. Many partially resemble river valleys on the Earth. However, the lunar rilles usually flow away from small pit structures. Also, the lunar samples indicate that the Moon has always been bone dry. Thus, the sinuous rilles probably mark lava channels or collapsed lava tubes that formed during mare volcanism. Still, in some cases, the lunar flows may have melted their way down into older rocks, much like rivers cut into their flood plains on Earth. Similar lava channels and tubes are found in Hawaii, but these are all much, much smaller than those found on the Moon.

Hadley Rille (from Orbit)

This photo shows the Hadley Rille on the southeast edge of Mare Imbrium. It is fairly well known because Apollo 15 landed there (see next image). The rille begins at the curved gash in the bottom left corner, and is clearest in the rectangular, mare-floored valley shown here. In the upper left, it gets much shallower and it slowly fades out of sight in Palus Putredinis. In all, the rille is over 75 miles (120 km) long. It is up to 5000 feet (1500 m) across and is over 950 feet (300 m) deep in places. It formed nearly 3.3 billion years ago . In contrast, lava channels on Hawaii are usually under 6 miles (10 km) long and are only 150 - 300 feet (50-100 m) wide. This contrast in channel size probably reflects (1) differences in the volume of erupted lava and (2) the difference in gravity. Note -- The bright bumps surrounding Hadley are peaks of the Montes Apenninus. These mountains mark the edge of the impact basin holding Mare Imbrium. They rise from 6000 to 15,000 feet (1800 - 4500 m) above the mare. (Apollo 15 image M-1135, arrow marks landing site of Apollo 15. Image taken from NASA SP-469, Geology of the Terrestrial Planets)

Hadley Rille (from Surface)

This is a photo of Hadley Rille from the lunar surface. Astronaut James Irwin is standing in the foreground by the lunar rover. At this point, the rille is nearly a mile wide (1.6 km). Note the many boulders on the Valley floor. These rocks have apparently rolled off of the Valley wall over time. Thus, any lava flows inside the rille are now buried by an unknown amount of rocks and soils. The rille appears to have cut down through a couple of older lava flows, however. These flows are probably the only actual bedrock seen by astronauts on the Moon. They form small cliff faces near the top of the rille wall in the distance. Due to the loose soil and steep slopes above these cliffs, however, no samples were taken of these units. (Apollo Photograph AS15-84-11450)

Posidonius Rille

This image shows a sinuous rille located in the crater Posidonius. The source of this rille is a shallow pit on the northern crater wall (arrow). The rille then meanders along the edge of the crater floor for over 60 miles (100 km). Finally, it reaches a hole in the crater rim and vanishes into Mare Serenitatis. Note that the rille is also only the last stage of mare volcanism within this crater. It lies on top of an older, smooth mare unit which buries part of the crater floor and much of the western crater wall. ( The black bar in the lower right is about 6 miles (~10 km) long.) (Lunar Orbiter image IV-86-H3)

Lava Tubes

This image shows a chain of small pits and ridge segments. It marks the likely site of a partially collapsed lava tube in western Mare Imbrium. Here, instead of a surface lava channel, lava flowed through a buried tunnel in the mare. After the eruption stopped, the tunnel then emptied. Where the roof of the tunnel has fallen in, we see pits. If the entire roof had fallen in, we might see a sinuous trough like other lunar rilles. Thus, it is believed that some lunar rilles mark collapsed lava tubes. These rilles are usually near other collapse pits, and some also merge with ridge-like features. Thus, uncollapsed lava tubes may still exist near some of these lunar sinuous rilles. (Lunar Orbiter image V-182-M, from Wilhelms (1987) The Geologic History of the Moon, USGS Prof. Paper 1348.)

Schroter's Valley 1

This photo shows Schroter's Valley (arrow) and the Aristarchus Plateau. It lies between Mare Imbrium and Oceanus Procellarum. This valley is the largest (widest) sinuous rille on the Moon. It also has a smaller rille inside it (see next photo). Both rilles come from the same vent, but they probably reflect two separate eruptions. Both rilles fade away into the plains of Oceanus Procellarum. (Apollo 15 image M-2611, from Wilhelms (1987) The Geologic History of the Moon, USGS Prof. Paper 1348.)

This is a detailed photo showing part of Schroter's Valley (see last image). It clearly shows a complex, highly contorted rille inside the Valley. Like many rivers on the Earth, this rille has many tight loops along its course. These loops are meanders, and they suggest a long-lived flow on a fairly flat surface. They also require active erosion of the valley floor. This channel may have partially melted its way into older lava flows. (Part of Apollo 15 image P-341.

Cones and Domes

While there are no large volcanoes on the Moon, a few smaller volcano-like features have been recognized. These features are mostly fairly small. Few are more than a few thousand feet (few hundred meters) high, or more than 6 to 10 miles (10 to 15 km) across. They are also somewhat irregular in outline, and most are not very striking in appearance. Few show any large central pit or vent structures, but many do have very small central pits or craters.

These lunar constructs resemble small cinder cones and volcanic domes on the Earth. However, such cones and domes may form differently on the Earth and Moon. On the Earth, cinder cones form when small explosive eruptions pile up pieces of lava around a central vent. On the Moon, however, such eruptions will throw things much further, leaving little to pile up near the vent. Instead of a volcanic cone, such lunar eruptions should form a broad, thin layer around the central vent (a dark mantling deposit). Similarly, lava domes on Earth form from very thick, pasty lavas. Basaltic lavas are more liquid, and tend to form broad, flat lava flows. On the Moon, most of the domes and cones appear to be made of basalts. Thus, they can not have formed like Earth domes from thick, non-basaltic lavas. Instead, the lunar domes/cones may mark places where the erupted basalts were just barely molten.

Marius Hills

Shown here are the Marius Hills in Oceanus Procellarum. These hills are the largest group of volcanic cones and domes on the Moon. Note the fairly uniform size and appearance of these features. Also note the sinuous rilles which have formed within the reg ion. (Lunar Orbiter image IV-157-H2, from Wilhelms (1987) The Geologic History of the Moon, USGS Prof. Paper 1348.)

Rumker Hills

This image shows the Rumker Hills in northern Oceanus Procellarum (arrows). These hills are low, flat mounds which formed on a small plateau. Their age is poorly known. Note the vary small central pits in the hills near the image center. (Lunar Orbiter image IV-170-H2, from Wilhelms (1987) The Geologic History of the Moon, USGS Prof. Paper 1348.)

Gruitheisen Domes

This image shows two mare domes in northwestern Mare Imbrium. They are located on the rim of an old, mare-filled impact crater. Note -- These domes differ in color both from mare basalts and from typical highland rocks (see mare types). They may mark a rare instance of non-basaltic lunar volcanism. (Lunar Orbiter Image V-182-M, from Wilhelms (1987) The Geologic History of the Moon, USGS Prof. Paper 1348.)

Dark Mantling Deposits

Although the mare formed from large effusive lava flows, there is some evidence for explosive volcanism on the Moon. In places, the lunar surface is covered by dark layers of material. The largest of these areas are near the edges of the lunar mare. They cover many thousands of square kilometers. They also include a range of knobs and other highland features. Thus, because lavas only flow downhill, these units can not be lava flows. Instead, they seem to mark areas where a thin layer has been draped over an older surface. Apollo 17 brought back samples from one such unit. They contain many small spheres of orange and black glass. These spheres probably formed from small drops of lava that cooled very quickly. Such droplets are thrown out of an eruption when bubbles of gas burst near the surface. Due to the size of the dark mantle deposits, however, some of these spheres may have been thrown hundreds of kilometers. Thus, despite the low gravity and lack of air on the Moon, some of lunar eruptions must have been quite violent. They may have resembled Hawaiian fire fountains, but on a much larger scale. There are also many smaller dark mantling units on the Moon. Most of these features are only a few kilometers in diameter. They are almost always located near the mare or in large crater floors. Many also lie along clear fault lines. Since most have a small central pit or crater, they are likely sites for small volcanic explosions. Some of these small eruptions may have released gases from shallow lunar intrusions.

Map of Large Dark Mantling Deposits

This is a map of the central nearside. It shows the sizes and locations of the largest known dark mantling deposits. Note that most of these units are near the edges of major maria. Some may in fact be partly buried by younger mare lavas. For reference, Apollo 17 landed in the Taurus Littrow unit on the southeastern edge of Mare Serenitatis. (This map is based on the work of Head, 1974 and Gaddis et al, 1985, as presented in Hawke et al, 1990.)

Sinus Aestum

This image shows one example of a large dark mantling deposit. It is located in Sinus Aestum on the central nearside, and is just east of the crater Copernicus. (Part of Copernicus lies on the leftmost edge of the photo.) Note how the dark areas in this photo (arrows) resemble the brighter highlands nearby. This suggests that the dark unit is relatively thin, and that it was dropped uniformly across an older, non-volcanic surface. (Earth-based telescopic photo from the Consolidated Lunar Atlas).

Crater Alphonsus

This oblique photo shows the crater Alphonsus on the eastern edge of Mare Nubium. Note the five dark patches along the crater floor edge. Each of these patches has a central pit. Three are located along clear fracture trends. Also, most of these pits are also elongated or irregular in shape. Thus, they are unlikely to be small impact craters. Instead, they probably mark the sites of small vulcanian eruptions. Note: The long corkscrew-like pole pointing into the image from the left is part of the Apollo Orbiter. (Apollo 16 image M-2478, looking southwards.)

Eruption Styles

This figure shows two styles of volcanism that might leave dark mantling units. (1) Strombolian eruptions feature bubbly, frothy magmas, and spatter large volumes of magma over large distances. Thus, such eruptions may have formed the largest dark mantle deposits on the Moon. (2) Vulcanian eruptions tend to be smaller than strombolian eruptions. They feature short explosions of gas and rocks. Because gases need to build up near the vent, they do not involve large volumes of magma. They also are somewhat episodic. Thus, vulcanian eruptions are more likely to form the smaller patches of dark materials on the Moon. They also are more likely to open up a recognizable central pit or vent structure. In the figure, red depicts new lavas in an eruption, blue represents old, colder lavas, and purple denotes mixtures of old and new lava. (figure modified from Wilson and Head (1981) J. Geophys. Res, vol. 86)

Mars

Major Sites of Volcanism

Overview

Mars has the largest shield volcanoes in the solar system. It also has a wide range of other volcanic features. These include large volcanic cones, unusual patera structures, mare-like volcanic plains, and a number of other smaller features. However, volcanic features are not common. There are less than 20 named volcanoes on Mars, and only 5 of these are giant shields. Also, volcanism occurs mostly within three regions. Even the mare-like plains cluster near these regions. The main cluster of volcanoes and lavas is in Tharsis. A much smaller cluster of three volcanoes lies in Elysium. Lastly, a few paterae are near the Hellas impact basin.

Differences from Moon

Age

Like the Moon, volcanism on Mars is very old. The mare-like plains on Mars are the same age as the lunar mare, roughly 3 to 3.5 billion years old. However, volcanism lasted much longer on Mars than on the Moon. It also seems to have changed over time. Volcanism in the highland paterae and mare-like plains on Mars stopped 3 billion years ago, but some of the smaller shields and cones erupted only 2 billion years ago. The giant shield volcanoes are even younger. These volcanoes formed between 1 and 2 billion years ago. The youngest lava flows on Olympus Mons are only 20 to 200 million years old. These flows are very small, however, and they probably represent the last gasp of martian volcanism. Thus, the odds of finding an active volcano on Mars today are very small.

Setting

Like the Moon, Mars shows no sign of plate tectonics. It has no long mountain chains, and there is no clear global pattern to the volcanism. Over half of Mars is heavily cratered like the lunar farside. Unlike the Moon, however, most martian volcanism lies outside large impact basins. Instead, the mare-like plains are mostly near the largest volcanoes. These plains also are not limited to the lowest elevations. Indeed, some lava plains are much higher than the cratered uplands. Lava plains may lie at lower elevations as well. However, thick layers of dust and sediment cover both the Northern Lowlands and the large basin floors. These layers reflect a long history of winds, glaciers and flood events. They also hide any volcanism that may have occurred in the low areas on Mars.

Processes

The concentration and duration of volcanism into these two regions are attributed to the evolution of a long-lived mantle hotspot.

Links

More Mars Volcano Information can be found at the "Geology of Mars" website curated by Albert T. Hsui, University of Illinois at Urbana-Champaign.

Giant Shield Volcanoes

Overview

The giant shield volcanoes on Mars are truly huge. The largest are three times as high as the biggest Earth volcanoes. They also are bigger in diameter. Thus, the biggest volcano on Mars is comparable to a pile of nearly 100 Hawaiian volcanoes. Despite this difference in size, the Mars shields look a lot like shield volcanoes on Earth. Both have the same broad flat profiles, large central calderas, and similar lava flow features. The giant martian shields are also much older than any Earth volcano. The youngest lavas on the martian shields are about 20 to 200 million years old. The oldest lavas are near 2.5 billion years old. Thus, these giant volcanoes were active for billions of years. This may explain their large size. On Earth, plate tectonics is always moving volcanoes away from their magma sources. Such movements are very slow, but they mean that most Earth volcanoes have distinct lifetimes. In the Hawaiian islands, for instance, volcanism lasts fo only a few million years on any given island. In contrast, the lack of plate tectonics on Mars allowed volcanoes to just keep growing. The only limit on their final size was the volume of lavas available.

Size Comparison

The giant shield volcanoes on Mars are much larger than any feature on Earth. Shown here is a graphic comparison of Olympus Mons to the two largest mountains on Earth. (Note: Mount Everest is not a volcano.) Olympus Mons is the largest and youngest volcano on Mars. It is nearly 27 km high, and about 700 km across. Its summit is nearly 72 km wide, which is close to the size of Mount Everest and Mauna Kea. Mauna Kea is the largest Earth volcano. It rises nearly 10 km from the ocean floor, and is about 120 km wide. Mount Everest is a plate tectonic feature, created by uplift and erosion in the Himalayan Mountains. It is about 9 km above sea level, and it sits in a mountain range roughly 200 km wide. (figure is from NASA SP-4212, p. 366, and has a 2x vertical exaggeration.)

Olympus Mons

This picture clearly shows how large and flat Olympus Mons is. Although the volcano is nearly 27 km high, it is over 20 times wider than it is tall. Thus, most of the volcano has a fairly gentle surface slope. The image also shows the distinct cliff which marks the base of Olympus Mons. In places, this scarp is up to 6 km high. In other places, it is hidden under lava flows cascading out into the surrounding lava plains. This cliff is unique among the giant shield volcanoes on Mars. The rough, crinkly patches around Olympus Mons are also unusual. They form the Olympus Mons Aureole.

Both the Aureole and the basal cliff are poorly understood. However, their origins may be related. In one theory, the basal cliff was formed by many large landslides. The Aureole marks material piled up at the bottom of these landslides.

Olympus Mons (from Orbit)

This picture clearly shows how large and flat Olympus Mons is. Although the volcano is nearly 27 km high, it is over 20 times wider than it is tall. Thus, most of the volcano has a fairly gentle surface slope. The image also shows the distinct cliff which marks the base of Olympus Mons. In places, this scarp is up to 6 km high. In other places, it is hidden under lava flows cascading out into the surrounding lava plains. This cliff is unique among the giant shield volcanoes on Mars. The rough, crinkly areas around Olympus Mons are also unusual. They form the Olympus Mons Aureole. Both the Aureole and the basal cliff are poorly understood. However, their origins may be related. In one theory, the basal cliff was formed by many large landslides. The Aureole marks material piled up at the bottom of these landslides. (Viking image mosaic from Carr et al, 1977, J. Geophys. Res., vol. 82, p. 3996.)

Olympus Mons (3D)

This image provides a perspective view of Olympus Mons. North is to the left of the image. Note the clear basal cliff and the gentle rolling slopes higher up the volcano. The faint radial texture above the basal cliff marks the traces of thousands of separate lava flows. To the left and lower right, we can also see places where lavas have flowed over the basal cliff.

Olympus Mons Caldera

This image shows the summit caldera on Olympus Mons. It is nearly 90 km long by 60 km wide, and it contains 6 overlapping pit craters. These craters resemble the calderas found on shield volcanoes on Earth. Thus, they are probably not eruption vents. Rather, they are probably collapse pits that formed in the roof of a deeper magma chamber. The size of these pits suggests that the magma chambers in Olympus Mons were fairly large. Likely diameters range from about 20 km to over 60 km. For comparison, calder as in the Hawaiian Islands range in diameter from ~3-5 km up to ~18 km. (Viking Orbiter image 890A68, from Lunar & Planetary Institute slide set Volcanic Features of Hawaii and Other Planets.)

Tharsis Montes

This image shows the three giant volcanoes known as the Tharsis Montes. Arsia Mons is in the lower left, Pavonis Mons is in the middle, and Ascreus Mons is in the upper right. Olympus Mons lies off the image to the upper left. These volcanoes lie in the center of the Tharsis region, and they form a line nearly 1500 km long. They are nearly 700 km apart, and each reaches nearly the same height as Olympus Mons (~25-27 km). These volcanoes are located on a large pile of lava flows , however, which is nearly 10 km high. Thus, the volcanoes are really only about 15 km tall. (Note: This is still more than half again the height of the Hawaiian volcanoes on Earth.) All three volcanoes seem to have formed together, and they were active for a very long time. Still, Arsia Mons appears to be slightly older than Pavonis Mons, and Ascreus Mons seems to be slightly younger. Therefore, volcanism in the Tharsis region may have slowly shifted north over time. A similar progression of volcanism is found in the Hawaiian Islands. Thus, the giant shield volcanoes on Mars may have formed over a mantle hotspot like that in Hawaii. (from digital mosiac of Viking 1 images, prepared for NASA by the U.S. Geologic Survey, published on the Mars CD-ROM VO_2014.)

Elysium Mons

This image shows Elysium Mons. This volcano is much smaller than the Tharsis volcanoes. It is only 9 km tall, and is about 240 km in diameter. Thus, it is nearly the same size as the largest Hawaiian volcanoes. Like the Tharsis Montes, however, Elysium Mons sits on a large pile of lava flows. This lets it rise 12 km above the mean planetary elevation. It grades so smoothly into the surrounding lava plains that its base is hard to see. A smaller volcano, Albor Tholus, can also be seen. It is partly buried by the lava plains surrounding Elysium Mons. Note the number of channels in this image. In places, these features look a lot like lunar sinuous rilles. They are all large flat-bottomed valleys. They begin abruptly in broad depressions. And their sources seem to form a ring centered on Elysium Mons. Like sinuous rilles on the Moon, these valleys might be lava channels. However, water is a more likely cause for their formation. Specifically, ground ice appears to have been widespread in the Elysium region. Such ice is easily melted near hot magmas. Thus, melt water provides a ready source for erosion in the Elysium region. Further, the loss of a lot of ground ice can cause collapse depressions near the channel sources. (from digital mosaic of Viking 1 images, prepared for NASA by the U.S. Geologic Survey, published on the Mars CD-ROM VO_2014.)

Volcanic Cones (on Mars)

Mars has a number of volcanoes with diameters between 50 and 150 km. These volcanoes are thus the same size as many large Earth volcanoes. Most are found within the Tharsis and Elysium regions. However, they are all older than the giant shield volcanoes. Thus, many have been partly buried by younger lavas. These volcanoes are divided into two general types. The first type are called "paterae," after the Greek word for a shallow saucer or bowl. They are fairly flat, they generally have little relief, and they often have large calderas. The second class of volcanoes are called "tholi." These volcanoes have much steeper sides, and they typically are taller than the patera structures. Both types apparently formed mostly from basaltic lava flows. Thus, they are much like Earth shield volcanoes. Their different appearances may mark either different depths of burial or changes in the nature of volcanism.

Biblis Patera

Biblis Patera is one of two volcanoes located near the center of Tharsis volcanism. With Ulysses Patera, it lies almost halfway between Olympus Mons and the southern Tharsis Montes. It is nearly 170 km long by over 100 km wide, and it has a central caldera nearly 55 km in diameter. It appears to be some 2 to 3 km tall. Biblis Patera is probably between 2 and 2.8 billion years old. However, it is surrounded by much younger lava flows. These flows come from Pavonis Mons to the east, and are clearest in the lower left of the image. The elongate shape of Biblis partly reflects the slope of these flows. Biblis is also clearly cut by a number of faults and graben. Note that some of these faults are buried by younger lavas along the edge of the volcano. This suggests that Biblis was modified by several episodes of tectonism and volcanism. (Viking Orbiter images 44B48 & 44B50, from Plescia (1994) Icarus, vol. 111.)

Ceraunius Patera

Ceraunius Tholus lies on the northeast edge of the Tharsis region. It is roughly 120 km long by 95 km wide, and it is about 2-3 km in height. It is probably close to 3 billion years old. Although running water formed the vast majority of channels on Mars, Ceraunius also shows two likely lava channels. One is the large channel in the upper center. Note the size of this feature. It is much larger than any of the other (water-carved) gullies on Ceraunius. It also appears to feed a volcanic cone in the old impact crater at its base. The second example is the chain of lunar-like pits in the left center. This pit chain links up to a clear channel downslope and probably reflects a collapsed lava tube. Both of these features strongly resemble the sinuous rilles on the Moon. (mosaic of Viking Orbiter images 516A24, 622A56, 622A58, 622A59 & 622A60, from Gulick and Baker (1990) J. Geophys. Res., vol. 95, No. B9)

Jovis Tholus

Jove's Tholus lies nearly due east of Olympus Mons and northwest of Ascreus Mons. Thus, it lies on the very northern edge of the lava plains surrounding the Tharsis Montes. It is roughly 80 km long by 60 km wide, and it has a large off-center caldera complex some 40 km in diameter. The total height is probably close to 2 km. Its age is very poorly constrained. The volcano could be anywhere from 2.3 to 3.5 billion years old. Like most tholi, Jovis Tholus is surrounded by younger lava flows. These lavas also cover a set of buried graben , however. The graben cut the easternmost and westernmost edges of the volcano. Jovis also shows signs of a long eruptive history. Its sprawling caldera contains no less than 5 craters. These calderas march southwest from the first, central caldera. Each younger caldera has a lower floor. (Viking Orbiter images 41B17 & 41B19, from Plescia (1994) Icarus, vol. 111.)

Ulysses Patera

Ulysses Patera lies just east of Biblis Patera. It is also located near the middle of Tharsis volcanism, and it has been buried by lavas from Pavonis Mons. It is about 100 km in diameter and is about 2-3 km tall. The central caldera is nearly 56 km across . Like Biblis Patera, Ulysses Patera is surrounded by younger lava flows. It is also cut by a few graben. The most striking features on this volcano, however, are two large impact craters. These craters overlap the caldera and clearly postdate all major volcanism at this site. Large craters are rare outside of the cratered uplands on Mars. Thus, the presence of two such craters on Ulysses Patera is highly unusual. This volcano appears to be very old. It has an estimated age of ~3.4 billion years. (VIking Orbiter images 49B68, 49B70 & 49B85, from Plescia (1994) Icarus, vol. 111.)

Uranius Tholus

Uranius Tholus is located just north of Ceraunius Tholus. It is ~57 km in diameter, and it shows a clear cone several kilometers high. It is heavily cratered and it seems to be quite old. It is probably over 3 billion years in age. It is also surrounded by fairly old volcanic plains, much older than those seen at Biblis and Jovis. These plains record a long history of faulting, partially seen at the upper left. Such fault systems have been buried closer to the giant Tharsis shields, but they may have strongly influenced the locations of both shield volcanism and of the smaller tholi. (Viking Orbiter image 516A23)

Highland Paterae

The highland paterae on Mars are unique. First, they are not part of the volcanoes in Tharsis and Elysium. They mostly lie in the Cratered Uplands far from other large volcanoes. They also are much older than the Tharsis and Elysium shields. Second, these paterae do not look like Earth volcanoes. There is no sign of actual lava flows. Rather, their central calderas are surrounded by sets of radial furrows. Third, these volcanoes are very flat. They typically are only 1-2 km high and 200-300 km across. These volcanoes are sometimes called ash shields. They seem to be (thin) piles of easily eroded volcanic ash. In contrast to the Earth, however, this ash seems to be composed of basalt. It probably formed when magmas met underground water and exploded into ash and steam. Such explosions help to explain the low height of these paterae. First, large ash eruptions tend to trap air beneath the ash clouds. This air helps support the ash and lets it spread out over wide areas. Second, Mars' gravity is about 1/3 the Earth's. Thus, an eruption on Mars can also carry ash much further than on the Earth.

Alba Patera

Alba is not a true highland patera, but it is also unique. It is the largest volcano on Mars, and is roughly 1600 km across. Despite its size, however, it is very flat. Alba has a total height of only ~3 km. Alba differs from both the giant martian shields and the highland paterae. Unlike the paterae, there are no signs of any furrowed ash deposits. Also, it is not in the highlands. Rather, it lies north of the Tharsis Volcanoes. Unlike the giant shields, Alba's volcanism is incredibly widespread. Its lava flows look like those on the other shields, but they are not piled as deeply. Also, it sits on a major fault trend that runs north northwards from the Tharsis region. As shown here, the youngest of these faults curve around to neatly frame Alba's summit. Alba also shows a long history of very fluid volcanism. There are both large sheets of mare-like lava flows and hundreds of long, narrow flows with a central channel or lava tube. Most of these flows are over 100 km long, and some are well over 300 km in length. Many are nearly 10 times larger than Earth flows which look similar. This increase in size probably marks larger and longer eruptions than those on the Earth. However, it may also need more liquid lavas than those on Earth.

Amphitite Patera

Amphitrite (AP) also lies on the edge of the Hellas basin. However, it is on the far side from Hadriaca. Although larger, it is also less striking in appearance than Hadriaca. The central ring structure is about 120 km in diameter. The furrowed ash shield is about 300 km across. It seems fairly thin, and it has little apparent relief. Like Tyrrhena Patera, it is located in a large unit of mare-like plains. Little else is known about the volcano.

NOTE: The circular feature PP may also be a highland patera. However, it shows no sign of any furrowed ash units. Thus, it may just be a large impact that was partly buried by plains lavas. (Viking orbiter images 94A74, 94A75, & 94A76, from Tanaka & Leonard (1995) J. Geophys. Res., v. 100).

Hadriaca Patera

Hadriaca is younger than Tyrrhena Patera. It is also larger and better preserved. The caldera in the upper left is nearly 60 km across. The furrowed deposits extend off the image to the lower left for over 300 km. However, they extend less than 100 km in the other directions. This is because Hadriaca lies on the edge of a large impact basin, and has mostly flowed down the side of this basin. It has a very low relief as well. Ignoring the slope into Hellas, it is only 1-2 km tall. This volcano also is linked to clear signs of martian ground water. It lies near the sources for a major martian outflow or flood channel. Several of these sources are seen to the right here as large, smooth-floored depressions. These features indicate that a lot of water or ice was buried near the volcano.

Tyrrhena Patera

Tyrrhena is the most striking of the highland paterae. It is about 300 km across, and it has a total relief of about 2 km. The central caldera is 12 km across, but it lies within a larger ring of fractures. This fracture ring may mark an older, buried caldera. It is 45 km in diameter, and it encircles the whole summit. The most striking features, however, are the broad furrows that run away from the fracture ring. These furrows are up to 200 km in length, and they suggest a period of heavy erosion. This erosion is very old and likely dates back to the time of volcano growth. It may mark a time of strong hydrothermal activity. Note that the furrows east of the summit are buried by a smoother set of volcanic deposits.

(Part of Viking Orbiter Mosaic 211-5213.)

Mare Volcanic Plains

Most of the extruded lava ended up forming the vast volcanic plains on Mars. About 60% of the Martian surface is covered by plains. Unlike the Moon or Mercury, one cannot definitively conclude that these are volcanic plains because they could be alluvial plains formed by hydro-processes or dust deposit plains formed by aeolian processes.One way to identify volcanic plains is by lava flow fronts. In this photo, one can see the advancing lava that filled a couple of ancient craters. Also note the smoothness of the lava plain and the rough ancient surface.This image shows the front of a lava flow advancing from the upper right hand corner. The rough, parallel ridges are probably the cooler upper portion of the flow that crumpled as the flow advanced. The front formed a cliff of about 30 meters (100 ft) high. Light-colored dust particles accumulated at the foot of the cliff to form the bright-colored region of the image The volcanic plains on Mars cover about 60% of the planet. It is thought that volcanism may have contributed to the formation of these features. Plain-style volcanism probably occurred throughout much of Mars’ geologic history. This can be seen in the resurfaced highland and lowland regions of the planet. To form the plains the lava flows would have probably had to have a high eruption rate. These features usually display wrinkle ridges that can be 10s km in length and 1000s m wide.

Other Volcanic Features (Mars)

Besides the large volcanoes, Mars also has many other volcanic features. The most obvious are the mare-like plains near Tharsis and the largest impact basins. However, the Viking mission found other, much smaller features as well. These include Earth-like cinder cones, a few small shields and some very old, rugged mountains in the cratered uplands. Some examples from each of these groups are shown here.

Cinder Cones 2

This image shows a field of larger cinder cones (arrows). Each is 2-5 km in size, and they are located near the edge of the cratered uplands. Like large cinder cones on Earth, most show a lop-sided, horse-shoe like pattern. Also note the many irregular pits within this image. These may be magma drainage features or a mark of volcanically melted ground ice within the region.

Cinder Cones 1

This image shows one likely group of small volcanic cones on Mars. The features are all less than a kilometer in size, and they are located in the northern lowlands. Each shows a clear central pit, and several lie on or along older faults (arrow). They are similar in size to Earth cinder cones, but the central pits are larger due to MarsÕ lower gravity. Like Earth cones, they mark the sites of small explosive eruptions.

Highland Volcano (?)

There are also some possible volcanoes in the cratered uplands. Because the cratered uplands are very rough, however, these volcanoes are very hard to find. This is one of the clearest examples. It is about 25 km in size, and it lies south of the Tharsis region. It is clearly an isolated mountain with an apparent (central?) caldera (arrow). It is also heavily furrowed and thus looks like a number of Earth volcanoes. The problem is that it has no clear lava flows. Thus, the inferred caldera could be just another small impact crater. Similarly, the peak itself might be just part of a very old impact crater. However, that seems unlikely in this case.

Small Shield

This image shows a small shield volcano in the Tempe region of Mars. It is about 5 km across (arrow), but it grades smoothly into a plains unit some 20 km in size. These plains may have erupted from the shield, but they also link up with other flows that came in from Alba Patera (upper left). The lava plains in turn bury a heavily faulted section of the old cratered uplands. These faults are part of a large, early deformation event around the Tharsis Montes. Note how both the shield and its summit fissure line up with the buried faults. (Viking Orbiter image 627A28, from NASA SP-460, The Geology of the Terrestrial Planets)

Plains with Calderas

Wind-blown sand and dust cover most surfaces on Mars. Thus, most of the mare-like plains on Mars show little sign of a volcanic origin. This is one of the exceptions. Shown here is a caldera located near the middle of Syrtis Major Planum. This plains unit is over 1200 km in diameter. Unlike the lunar mare, however, it does not fill the floor of a large impact basin. Rather, it covers the cratered uplands just outside the Isidis impact basin.

The caldera itself is 70 km across, and several hundred meters deep. It marks the collapsed roof of an old, shallow magma chamber. To the northwest, it is surrounded by faults and graben. To the southeast, the caldera wall has been buried by younger lava plains. Note the small volcanic cone (arrow) located on the caldera floor.

Valles Marineris Pyroclastics

This image shows a small part of the Valles Marineris Canyon System. The upper half of the picture is a canyon wall. The lower half is part of the canyon floor. The band of dark splotches running through the middle is a set of minor volcanic vents. These splotches are fairly thin, and they seem to be centered on faults within the canyon wall. They are also very young. They are younger than almost every other feature in the canyons, and some may be less than 100,000 years old. Thus, they may be the youngest volcanic units on Mars. They probably mark very small pyroclastic eruptions, much like the smallest dark mantling deposits on the Moon.

Io

Io, the innermost large moon of Jupiter, is about the same size and density as Earth's Moon. Io is the most volcanically active body known in the Solar System. Eruptions are so common and so large that the entire surface can be buried under 100 meters of material every 1 million years (it takes submarine volcanoes about 80 million years to resurface about two-thirds of the Earth). Impact craters, which are common on many planets and moons, are absent on Io because of the frequent volcanic eruptions bury them.

These enhanced (false) color views of Io highlight details of the surface. Some areas on Io are truly red and are closely associated with very recent explosive eruptions and volcanic plumes. The most prominent red oval surrounds the volcano Pele (far right). Galileo images courtesy of NASA's Jet Propulsion Laboratory. The following pages provide an overview of many aspects of Io's volcanoes.

Inside Io

Io has differentiated into layers. Understanding the number and composition of the layers is still being studied. Most scientist agree there is a core surrounded by mantle. A simple 2-layer (metallic core and a silicate mantle) model suggests the core is about 17-20 percent of Io's mass and has a radius of about half of Io's radius (Anderson et al. 1996). Another model (that assumes a pure iron core) suggests the core is 11-14 percent of Io's mass and has a radius of about one-third of Io's radius. Schubert (1997) used a three-layer model to suggest the presence of a thick (100-250 km) outer layer on Io. Several lines of evidence indicate that Io's metallic core is at least partly molten. Some models call for Io to have silica-rich crust about 40-60 km thick. The crust would be made of alkali-rich minerals, probably feldspars and nepheline. Much of the mantle may be pure forsterite (magnesian-rich olivine).

The enormous gravitational forces of Jupiter cause heating within Io. Most of this heating is concentrated in the asthenosphere, estimated to be 50-100 km thick. Additional heating occurs deep in the mantle. Melting is probably located at the base of the lithosphere. Based on the amounts on energy released by Io's volcanoes each part of the interior has probably been remelted at least 100 times over the satellite's history.

Landforms

Two Galileo images of Io. These images reveal that the topography is very flat near the active volcanic centers such as Loki Patera (the large dark horseshoe-shaped feature near the top right edge in the left-hand image) and that a variety of mountains and plateaus exist elsewhere. Image courtesy of NASA/JPL.

The surface of Io has three common features:

Mountains

The mountains of Io are rugged and isolated. They are separated by the plains and the mountains and plains cover about 2% of the surface. Individual mountains can be up to 100 km long and have relief of 9 km. The mountains do not appear to be volcanic in origin (although they are mantled in sulfur) and are thought be older than both the plains and volcanoes. The mountains have been modified by both tectonic and erosional processes. The presence of the mountains suggest Io has a rigid lithosphere, possibly up to 30 km thick.

Plains

About 40% of Io is covered by plains with low relief and light and dark areas. The plains are probably layers of pyroclastic material erupted from volcanoes and possibly lava flows of different compositions or ages. Layering can be seen on the edges of some plains. Other plains contain plateaus with smooth tops and escarpments from 150 to 1700 m high. The escarpments are evidence for erosion.

Volcanoes

Only about 5 percent of Io is covered by volcanic vents. About 500-700 volcanic centers have been identified but, over the last decade, most of the energy has been released at only four centers. The energy is released from these centers at enormous rates. Carr (1997) reported that 356 calderas had been identified in the Voyager and Galileo coverage. The largest volcanoes have diameters of more than 250 km and are closer to the equatorial region. Volcanoes at higher latitudes tend to be smaller, less than 100 km in diameter. The random distribution of Io's volcanoes suggests a lack of mantle convection, which is partially responsible for linear hot spot tracks and island arcs on Earth.

Paterae, low-profile volcanic shields are the most common type of vent. Their flows can cover large areas and reach lengths of 700 km. Such long flows suggest high eruption rates and/or low viscosity material. Some patera have summit calderas with relief of 1-2 km from floor to rim. The images above compare the 1979 Voyager 1 image of Loki Patera with Galileo images taken in 1996. The patera is at the center of the images. A dark fissure is just above and right of the patera. Voyager observed an eruption from this fissure in 1979.

Ra Patera covers an area of 760 x 480 km and has numerous long, narrow flows that radiate from the summit. These views of Ra Patera show changes seen on by Voyager 1 (upper left and upper right), Galileo (bottom right), and Voyager 2 (bottom left). The Galileo images reveal the detailed morphology of new deposits. Dark materials are interpreted as the overflow of lava flows from the caldera. New bright deposits, also though to be lava flows, cover an area of about 40,000 square kilometers and surround the dark materials. Images courtesy of The Jet Propulsion Laboratory and NASA. Maasaw Patera, another shield volcano with summit caldera, has been compared to Volcan Alcedo in the Galapagos. The patera are thought to be made mostly of silicate lava flows with interbedded sulfur lava flows and pyroclastics.

Io may have calderas with active lava lakes and fissures erupting silicate lava flows. These calderas may be up to 200 km in diameter and are located on the surface of the plains. Volcanic plumes originate in some calderas.

Eruption Styles

Explosive eruptions have been observed on Io and there is indirect evidence for effusive eruptions.

Explosive

Galileo color images showing two volcanic plumes on Io. A plume erupting over Pillan Patera was captured on edge of the moon (see main image and inset at upper right). The plume was 140 kilometers (86 miles) high. The Galileo spacecraft will pass almost directly over Pillan Patera in 1999 at a range of only 373 miles (600 kilometers). The second plume is erupting over Prometheus, seen near the center of the moon and near the boundary between day and night and the inset at lower right. In the inset image, the shadow of the plume can be seen to the right of the vent. The plume is about 45 miles (75 kilometers) high.

Two types of eruption plumes have been observed: Prometheus-type and Pele-type.

Prometheus-type

Pele-type

Plume heights

50-120 km

up to 300 km

Plume character

opticallythick, dark jets

opticallythin(transparent)

Deposits

bright halos, 200-600 km diameter

dark halos, 1000-1500 km in diameter

Eruption Velocities

about 500 m/s

up to 1000 m/s

Duration

months to years

days to months

Location

common near equator

restricted longitudes

Temp.of associated'hot spots'

about 450 K

about 600 K

Ejection velocities for explosive eruptions are estimated to be 500 to 1,000 meters per second. Plume diameters can be as much s 1,000 km. In December 1996, Pele's plume had at a height of 460 km. Most of the plume-producing eruptions are near the equator (between 30 degrees north or south). Two of the eruption sites, called Pele and Loki, are associated with calderas. Explosive eruptions can continue for at least a few days but some wane after a few hours. Sulfur dioxide gas may be the driving force of the explosive eruptions.

Effusive

High-resolution image of part of Io showing lava flows and other volcanic features on Io.

Earth-based monitoring of thermal emissions on Io have been interpreted as eruptions of surface lava flows. In 1996, two effusive eruptions produced about 3 square km of lava at eruption rates of 10,000 to 1,000,000 square meters per second. Eruption temperatures were greater than or equal to 1130C (Strawberry and others, 1997). Eruptions on Io may produce pahoehoe and aa flows, possibly as overflow from lava lakes or from fissure eruptions.

Sulfur vs. Silicate

The size and density of Io are about the same as the Earth's Moon. Since pieces of the Moon have been directly sampled and found to be made of silicate minerals (minerals with silicon and oxygen), scientists have suggested that most of Io is also made of silicates. In contrast, the red color of Io (the reddest object in the Solar System) and spectra from the surface indicate that sulfur is present.

Sulfur

Sulfur is an unusual substance. It boils at temperatures higher than about 275 C and can remain molten down to 120 degrees C (lava flows made mostly of silica solidify at about 1000 degrees C). Thus, magmas on Io can be generated at much lower temperatures and lava flows can remain molten to much lower temperatures.

The variation of viscosity with temperature for molten sulfur is also unusual:

Viscosity (poises)

Temperature ( C )

Color

50-500

275-220

black

500-1000

220-175

red

1000-1

175-150

red-orange

1-0.08

150-140

orange

0.08-1

140-50

yellow

Based on a graph in Rothery (1992).

As temperature decreases the viscosity initially increases. At about 175C, the viscosity drops dramatically almost 4 orders of magnitude while the temperature drops only 40C. This means that lava flows actually get more fluid as they cool (the opposite is true of the silicate lava flows of Earth).

All of the colors listed above have been observed on Io and are best explained by the eruption of sulfur. The color changes with cooling help to map the eruption temperatures of volcanic products on Io. Surface compositions probably consist of: sulfur at various temperatures, anhydrous mixtures of sulfur allotropes with sulfur dioxide frost, and sulfurous salts of sodium and potassium.

Silicate

Some planetary geologist believe silicate AND sulfur volcanism occur on Io. The presence of mountains with 9 km of relief suggests silicate material is involved because sulfur and its compounds does not have enough strength to support such features. Likewise, the relief along the edges of plains and within patera calderas could not develop if the surface material was all sulfur. The large size of the calderas on Io requires that the crust is strong and at least 10-20 km thick. A more likely scenario is a thin veneer of sulfur or sulfur compounds over a crust of silicate rocks. Silicate volcanism is probably high-volume low viscosity (basalt) lava flows erupted from low shields and possible fissures.

The presence of hot spots (not to be confused with hot spots on Earth) also supports the presence of silicate volcanism. Hot spots are active or recently active volcanic regions on Io. They are recognized by high thermal emissions. Using a Near-Infrared Mapping Spectrometer (NIMS), 30 hot spots have been detected. The NIMS image on the left shows Loki Patera on February 21, 1997, as Galileo made its sixth orbit. The image on the right was taken on March 12, 1997, using the Infra-Red Telescope Facility (IRTF) on Mauna Kea, Hawaii. The image shows the enormous amount of heat generated at Loki during an eruption.

The Galileo image on the left shows volcanic hot spots on Io's darkside. Io was in Jupiter's shadow when the image was taken. This is the highest-resolution image ever acquired of hot spots. The mosaic of Voyager images on the right shows the locations of the hot spots seen in the Galileo image. Image courtesy NASA/JPL.

The hot spots (volcanic centers) are named after mythological figures associated with fire and thunder: Janus, Hi'iaka, Zal, Gish Bar, Sigurd, Monan, Altjirra, Amirani, Maui, Malik, Tupan, 9606W, Prometheus, Culann, Zamana, Volund, Aidne, Fo, Sethlaus, Rata, Lei-Kung, Isum, Marduk, 9611A, Kurdalagon, Mulungu, Pillan, Pele, Daedalus, W. Pele, and Loki. Lopes-Gautier and others (1997) lists the latitude and longitude of these hot spots. The temperatures range from about 100 to 333 degrees Celsius over areas of 192 to 3 square km. All of these hot spots are within 50 degrees of the equator. Hot spots at higher latitudes, if the exist, may be detect by later orbits of the Galileo probe. Ten of the hot spots detect in 1979 were still active in 1997.

High resolution images of hot spots using Galileo's CCD system are interpreted to be caldera floors (actively convecting lava lakes) and/or possibly pahoehoe lava flows. The small areas are thought to have temperatures of at least 725 degrees Celsius or higher (Note: these temperatures are higher than the boiling point of sulfur in a vacuum). These images are some of the best evidence for active silicate volcanism (possibly basalt) on Io.

Studying Io

Voyager 1 and Voyager 2

The Voyager probes obtained images of about 35% of Io at a resolution of 5 km. In some areas resolution was as good as 0.5 km. These images allowed geologic maps of Io to be constructed. Geologists could recognize mountains, plains, and volcanic vents, and the relative ages of these features. Nine eruption plumes were discovered during the Voyager 1 mission. Voyager 2 arrived four months later. Voyager 1 image taken on the morning of March 5, 1979 at a range of 377,000 kilometers (226,200 miles).

Future observations: Io is the most volcanically active body in the solar system. Scientists hope to learn more about the fiery satellite when Galileo continues its studies over the next two years, during a mission extension known as the Galileo Europa Mission. The extended mission will include eight additional encounters of Europa, four of Callisto, and two close Io flybys in late 1999, depending on spacecraft health. Galileo will pass very close to Pillan Patera in the first of the two Io flybys, so high- resolution images can be acquired over a small portion of this area.

Earth-based Telescopes

The NASA Infrared Telescope Facility (IRTF), on Mauna Kea, Hawaii, collects infrared images of Io when the satellite is observable. This data set is in support of images collected during the Galileo mission. The images can detect eruptions and, under the right conditions, the active volcano. This telescope provides better time coverage and time resolution of volcanic activity on Io compared to Galileo observations.

The images from the telescope have detected very hot events (>1220 C) that lasted days or weeks (Spencer and others, 1997). Some events are located at known calderas. Loki, Io's most powerful single volcano, appears to have periods of increased activity that last several months. Interestingly, not all hot events are associated with plumes or surface changes.

The Infrared Telescope Facility at Lowell Observatory in Arizona has detected thermal emissions on Io caused by violent silicate eruptions, possibly from fire fountains or the overflowing of lava lakes.

Hubble Space Telescope

The Hubble Space Telescope (HST) has been used to observe Io. For example, between March 1994 and July 1995 a major brightening (eruption) was observed at Ra Patera. HST has also observed eruption plumes from Pele such as the one shown in the above image. Plume height is about 400 km. These observations allow estimates of the plume density, composition, and mass and detection of rapid changes in plumes.

Carlson, R.W., and others, 1997, The distribution of sulfur dioxide and other infrared absorbers on the surface of Io, in, Special Section: Io Volcanism in the Galileo Era: Geophysical Research Letters, v. 24, no. 20, p. 2479-2483.

Davies, A.G., and others, 1997, Temperature and area constraints of the South Volund Volcano on Io from NIMS and SSI instruments during the Galileo G1orbit in, Special Section: Io Volcanism in the Galileo Era: Geophysical Research Letters, v. 24, no. 20, p. 2447-2450.

Spencer, J.R., and others, 1997, A history of high-temperature Io volcanism: February 1995 to May 1997, in, Special Section: Io Volcanism in the Galileo Era: Geophysical Research Letters, v. 24, no. 20, p. 2451-2454.

Titan

Volcanism on Titan

The Cassini Titan Radar Mapper obtaines Synthetic Aperture Radar images of Titan's surface which show that Titan's surface is very complex geologically, showing evidence of major planetary geologic processes, including cryovolcanism.

The paper below discusses the variety of cryovolcanic features first identified from SAR images, their possible origin, and their geologic context. The features which we identify as cryovolcanic in origin include a large (180 km diameter) volcanic construct (dome or shield), several extensive flows, and three calderas which appear to be the source of flows. The composition of the cryomagma on Titan is still unknown, but constraints on rheological properties can be estimated using flow thickness. Rheological properties of one flow were estimated and appear inconsistent with ammonia-water slurries, and possibly more consistent with ammonia-water-methanol slurries. The extent of cryovolcanism on Titan is still not known, as only a small fraction of the surface has been imaged at sufficient resolution. Energetic considerations suggest that cryovolcanism may have been a dominant process in the resurfacing of Titan.